In English, one common use of a suffix is to change the intensity of a word. Note, for example, the difference between "happy," "happier" and "happiest."
In Spanish, suffixes are frequently used to make something more endearing (un gato is a cat, but un gatito can be a cute cat or one dear to the speaker) or less so. It is also common in Spanish for one word to be used with a variety of suffixes to arrive at different meanings. For example, a caja is a box, but a cajetilla can be a packet and a cajón a drawer.
In both languages, suffixes are used in verb conjugation and in making words plural. They can also be used to change a word to a different part of speech. For example, adding the suffix "-ly" (the equivalent of -mente in Spanish) to the adjective "careful" (cuidada) makes it the adverb "carefully" (cuidadamente).